I've been asked twice lately which modules should be put on a couple tanks in the Waffle line of TDs (Nashorn, Pz. Sfl. etc.) I'll tell you how I use modules.
First off I think how will the mod perform on a particular tank. Well you have some things to take into consideration first. i'll use my tanks for examples. First off I don't put mods on any tank T7 and under. These tanks already break even at worst in every match. so it's somewhat useless to put mods on. Also keep in mind that to remove a mod it will cost gold, so I only put mods on the tanks I plan to keep for a while.
Now for the mods. Once again i'll use my tanks to illustrate. my Rhm.B.WT has a rammer, binocs, and a camo net. the reasoning is this. The Rhm is a TD so it makes a good ambush weapon, but with the gun it makes an excellent sniper tank for maps such as El Halluf. This is where the mods come in. As a sniper I need to see way out there, the binoc mod does just that, it extends my view to 500m. Now that is just fine and all but it's still not good enuff. this is where your crew skills come into play. In this tank I have all of my crew trained to 100% on camo. As a TD I camp a lot, so sitting in a bush with a camo net helps me hide better, but when you add in the camo of the crew skills, now you can almost drive past me without seeing me. The crew skills and the camo net are all cumulative, in other words they all work in conjunction with each other to lower your chances of being seen. now I have my commander training skills that extend my view range, which is cumulative with my binoc mod. I have my loader running skills that will help his load time as he advances. Once again the rammer, and the loader skills are cumulative. My other crew members are running skills that will help in other areas to make the tank perform better.
The reason i'm using my Rhm for an example is because it IS the perfect example for this post. See i'm using the Rhm as an ambush vehicle and a sniper vehicle so I have geared all of my crew skills for those purposes and I have used mods that compound the effects of those skills.
So in essence when deciding on mods first you need to decide what purpose the tank will serve. is it a scout? On my scount I have the mod that gives me 10% more view range, I don't need the binocs because they are for stationary use where the 10% mod is great cuz when i'm scouting I can pick up an enemy a little farther out and i'm not sitting still to snipe.
I hope you get a better understanding of mod use from this and if you have any questions please ask